1. Field of the Invention
The present invention relates generally to a DC-DC converter and in particular concerns a DC-DC converter in which power loss and accompanying heat generation due to switching operation is reduced to a minimum.
2. Description of the Prior Art
In FIG. 1 of the accompanying drawings, there is shown a circuit diagram of a conventional DC-DC converter. In this DC-DC converter circuit, a DC voltage E of a DC power supply source 1 is intermittently applied to the primary winding of a transformer 3 through repeated turn-on and -off operation of a switching circuit 2, whereby a secondary voltage is induced in a secondary winding of the transformer 3 under the influence of changes in the magnetic flux. The secondary voltage thus induced is rectified by a diode 4 and supplied to a load 6 after having been smoothed by a capacitor 5. In the operation of the DC-DC converter described above, a voltage e appearing across the switching circuit 2 and a current i conducted therethrough are of such waveforms as shown in FIG. 2 at (a) and (b), respectively. When changes in the voltage e and the current i in the on-off operation of the switching circuit 2 are traced, a closed loop such as illustrated in FIG. 3 is obtained. The profile of e-i loop is generally of a circular or angular form depending upon the leakage inductance of the transformer 3 and the capacitance of the capacitor 5, and either one of areas, defined by turn-on transition curve and by turn-off transition curve, prevails over the other in size and is fairly large. Thus the power loss, represented by the power product e.i, causes instantaneous heat generation in transistors and other elements constituting the switching circuit 2, degrading the operation efficiency of the DC-DC converter on the whole, to a disadvantage. This unwanted phenomenon may be explained by the fact that the turning on and off of voltage can not be separated from that of the current but both proceeding simultaneously.
As a typical example of the technical fields to which the DC-DC converter according to the present invention may be applied, a digital communication system in which the DC-DC converter is incorporated as a power supply source for each of subscriber or terminal apparatus will be briefly described.
In general, in digital communication over the subscriber's lines, each of the terminal apparatus is connected to a station through two wires, one of which serves to supply a DC power to the terminal apparatus from the station, while the other serves for transmission of digital signals. Assuming that a digital signal to be transmitted to the terminal or subscriber apparatus from the station is represented, for example, by 2 V.sub.o-p, this signal undergoes attenuation down to about 10 mV.sub.o-p at the terminal apparatus in the case the latter is located about 5 km from the station, as is well known in the art. The terminal station has to extract digital or logic values "0" and "1" from the attenuated digital signal 10 mV.sub.o-p. In this situation, it is noted that when the DC-DC converter is used as the power supply source for the terminal equipment, the rate of voltage change (dv/dt) in the switching operation of the converter is very large and thus may bring about noise as large as 500 mV.sub.o-p, which makes it impossible to extract the logic values from the digital signal as mentioned above with an acceptable accuracy. Theoretically, the noise may be reduced by smoothing the voltage change (dv/dt). However, in that case, the power loss, as apparent from the description above in conjunction with FIG. 2, will tend to be increased, bringing about considerable heat generation in the switch circuit as well as degradation in the operation efficiency of the DC-DC converter, to disadvantages.